The invention relates to an apparatus for monitoring the respiration and cardiac activity of a person, especially but not exclusively a small child, lying in a bed, and which includes a sensor responsive to pulses originating from the person being monitored.
Such an apparatus is described in the paper "An indirect method for the continuous recording of respiration" by H. U. Thal, Dusseldorf University, 1973. According to this paper, piezoelectric load cells are built into the legs at the foot of the bed. The inhalation and exhalation of the patient results in a displacement of the center of gravity so that the feet of the bed are accordingly subjected to varying loads. These loads are converted to electrical signals by the load cells and these signals are amplified to produce output signals. In this manner, there are obtained simultaneously a so-called ballistocardiogram which, at a corresponding frequency, is characteristic of the cardiac activity, and a respiratory curve which, at a lower frequency, indicates respiratory activity. In this case, the signals originating from the cardiac activity are removed by means of a corresponding filter in order to obtain clear information about the course of respiration. This type of monitoring is of considerable importance not only in the intensive care unit of a hospital but also in a ward for new-born babies because in this manner, by appropriate use of the output signals, indications of respiratory arrest can be recognised immediately so that the necessary counter-measures can be taken. The advantage of this method is that it is possible to monitor respiratory activity without having to undertake any measurements on the actual patient or new-born baby. Further possibilities of monitoring on this basis are, however, neither anticipated nor possible.
In contrast, the problem underlying the invention is to obtain more extensive information about the respiratory and cardiac activity than is possible with this known apparatus. It is desirable therefore to have, at least in addition to information about respiratory activity, continuous information about cardiac activity on the basis of reliably usable signals. It is also desirable for the information to be extended in the direction of a more highly differentiated monitoring of both respiration and cardiac activity. In the case of respiration, additional details about the type of respiration (abdominal or chest respiration) and the respiration pressure are of interest, while in the case of cardiac activity the direction of blood circulation, especially in new-born children, deserves increased observation. For example, passage of blood through a still open ductus arteriosus would give a characteristically directed pulse. In connecting the pulmonary artery with the descending aorta, the ductus arteriosus forms an angle of less than 90.degree.. Different pulse components would be produced depending on whether the patient was lying on his back or abdomen or on his right or left side.